ASPIRE Winter 10 - Aspire - The Concrete Bridge Magazine

The 50-ft simple-span Pi girders are supported
on plain neoprene bearing pads.
Due to the high costs of upgrading and
modifying the forms, the sole fabricator
interested in casting the modified Pi
sections delivered a bid that was too high
for the budget. FHWA officials at TFHRC
suggested that further revisions be made
to the first-generation section and new
forms be created for a second-generation
Pi girder. The FHWA agreed to fund the
forms and purchase two test beams for
evaluation. The three production beams
would be purchased at the same time
to reduce setup and casting costs for all
of the beams. In addition, the revised
section would be available for use on
future projects by other state agencies.
This approach was taken, leading to four
key revisions being made to the firstgeneration
section:
1. Two types of fillets, 5 in. and 8 in.
deep, were added at the web-todeck
connection to improve concrete
flow during placement and to stiffen
the slab section.
2. The interior deck thickness between
the webs was increased to 4 1 /8 in. to
reduce service load stresses.
3. The web spacing was reduced by
4 in. to provide a more balanced
spacing of the webs for the threebeam
cross section and to reduce
service load stresses.
4. The post-tensioning was removed
from the deck. Due to the lack of test
data on the revised section, No. 5
reinforcing bars at 1-ft centers were
included in the deck.
Two 25-ft-long test beams were cast first,
followed by three 51-ft-long production
beams. The three bridge beams were 8
ft 4 in. wide and 2 ft 9 in. deep with two
tapered webs about 3 in. thick spaced at
4 ft 5 in. Deck thickness was a constant
4 1 /8 in. between the webs and a tapered
thickness outside the webs from 6 7 /8 in. to
5 1 /4 in. at the edge of the slab. Flanges at
the bottom of the beam webs were 7 in.
deep by 1 ft wide. Each flange contained
nine 0.6-in.-diameter strands tensioned to
72.6% of ultimate. Total concrete quantity
was 11.3 yd 3 of UHPC per unit.
Ready-Mixed Concrete
Trucks Used
Typically, high-speed pan mixers are used
because of the large amount of time
and energy needed to thoroughly mix
the concrete. In this case, ready-mixed
concrete trucks were used for mixing the
required 21,500 psi design compressive
strength. As the material’s performance
is affected by the alignment of the steel
fibers, a horizontal bucket almost as wide
as the form was fabricated to place the
material so it would flow freely along the
form and properly align the fibers.
The beams were cured in two stages.
The first stage involved curing at ambient
temperatures, although steam curing up to
115 °F could be used in a similar manner to
curing precast, prestressed concrete beams.
The Pi girders were covered with plastic and
kept at ambient temperatures until matchcured
cylinders indicated a compressive
strength of 5100 psi had been achieved.
Then the forms were opened but left in
place to allow for shrinkage of the section.
Curing at ambient temperatures continued
until the compressive strength of matchcured
cylinders reached 14,500 psi. The
forms then were removed and the strands
were detensioned.
The second curing stage began with
thermal treatment applied to the UHPC
beams with moisture present. The goal
was to achieve a temperature of about
190 °F along with relative humidity of at
least 95% for at least 48 hours. Thermal
treatments have been shown to enhance
not only the members’ strength but
their durability as well. The beams were
wrapped with insulating tarps, and steam
was injected underneath the girders. The
temperature was increased gradually over
a period of approximately 6 hours. Once
the second curing period was completed,
the curing temperature was decreased
gradually over a period of approximately
6 hours.
The beams were fabricated in September
2008, while the contractor began
mobilization, grading, and substructure
work. The beams were erected in mid-
October, and the concrete for the end
spans was placed a few weeks later. The
project was completed in November,
requiring 52 days from start to finish.
Waffle Slab Project
Work with UHPC continues, with a third
bridge project now under development
with Coreslab Structures (Omaha) in
Bellevue, Neb., for use in Wapello
County, Iowa. This project will use the
UHPC mix in a precast concrete deck
on a single-span, prestressed concrete
beam bridge. To optimize the material,
the deck panels will be cast with a waffle
shape. Component casting is scheduled
to begin in the winter of 2009-2010,
with construction to take place in the
summer of 2010.
By using UHPC in bulb-tee beams, the
optimized pi girder, and a waffle-shaped
deck panel, the project team will expand
the knowledge base and facilitate the
wider use of advanced cementitious
materials to solve specific transportation
challenges.
____________
Brian Keierleber, is county engineer for
Buchanan County, Independence, Iowa;
Dean Bierwagen, and Ahmad Abu-
Hawash, are with the Office of Bridges
& Structures of the Iowa DOT in Ames,
Iowa; and Terry Wipf, is director of the
Bridge Engineering Center at Iowa State
University in Ames, Iowa.
For more information on this or other
projects, visit www.aspirebridge.org.
26 | ASPIRE, Winter 2010